With the aid of stable isotope tracer methods, involving use of combined gas chromatograph-mass spectrometric techniques, we are exploring the adaptations of whole body protein and amimo acid metabolism in adult subjects. Studies include the effects of level and source of protein, amino acids and energy intake on whole body leucine, lysine and glycine and alanine metabolism. Concurrent use of 13C- and 15N-labels provides an opportunity to examine dynamic aspects and interrelationships among a number of amino acids, for a given dietary condition. These studies have shown that the specific mechanisms for achieving body amino acid homeostasis depend upon the range of protein intake studied; that is, they also appear to be integrated with the nitrogen and amino acid requirements of the individual whether in the submaintenance or surfeit range of intake.